A proteomics approach to the identification of mammalian mitochondrial small subunit ribosomal proteins

Mammalian mitochondrial small subunit ribosomal proteins were separated by two-dimensional polyacrylamide gel electrophoresis. The proteins in six ind ividual spots were subjected to in-gel tryptic digestion. Peptides were sep arated by capillary liquid chromatography, and the sequences of selected pe ptides were obtained by electrospray tandem mass spectrometry. The peptide sequences obtained were used to screen human expressed sequence tag data ba ses, and complete consensus cDNAs were assembled. Mammalian mitochondrial s mall subunit ribosomal proteins from six different classes of ribosomal pro teins were identified. Only two of these proteins have significant sequence similarities to ribosomal proteins from prokaryotes. These proteins corres pond to Escherichia coli S10 and S14, Homologs of two human mitochondrial p roteins not found in prokaryotes were observed in the genomes of Drosophila melanogaster and Caenorhabditis elegans. A homolog of one of these protein s was observed in D. melanogaster but not in C. elegans, while a homolog of the other was present in C. elegans but not in D, melanogaster, A homolog of one of the ribosomal proteins not found in prokaryotes was tentatively i dentified in the yeast genome. This latter protein is the first reported ex ample of a ribosomal protein that is shared by mitochondrial ribosomes from lower and higher eukaryotes that does not have a homolog in prokaryotes.